The present invention relates to a semiconductor integrated circuit device to be utilized in a color palette device generating color pixel signals for a color display.
A personal computer, such as a laptop type using a panel display such as a liquid crystal display or plasma display, is required as a part of its function to allow connection to a CRT (cathode ray tube) color display of a desktop type personal computer. The color palette device forming a color pixel signal for the CRT color display can not control a panel display as described above because it only outputs analog color signals of R (red), G (green), and B (blue) which may be subjected to digital/analog conversion. The pixel signal for driving the panel display is a digital color signal, and cannot utilize the analog color signals. Consequently, when the panel display is driven using the color palette device, which outputs only analog color signals, an additional circuit such as an analog/digital converter becomes necessary. This causes the number of parts of a board for color display to be increased and the circuit system constitution to be complicated.
Also, since a laptop type personal computer is often driven by a battery, it must have low power consumption. Consequently, the digital/analog converter to be used for the CRT color display must be at a low power consumption state when not being used. However, if the operation voltage of the A/D converter is interrupted completely it becomes difficult to use the converter in such a state and a rise in standby time of the circuit becomes necessary.
In "Transistor Technology", January 1987 published by CQ Publication Company on Jan. 1, 1987, reference titled "Design of ACRTC graphic board (1)" from page 402 to page 411, particularly at pages 410 and 411, a circuit diagram of ACRTC graphic board using a color palette device Bt-450 is disclosed. The board comprises two graphic video attribute controllers (HD63486CP) U5 and U6 for receiving frame data from sixteen frame buffer memories (HM504464) M1 through M16, for generating video outputs, a color palette device (Bt-450) U8, for receiving the video outputs at its inputs P0 through P3, and outputs IO.sub.R, IO.sub.G, and IO.sub.B connected to an analog connector P2, and a digital output connector P3 for receiving the video outputs. When the digital output connector P3 of the board is connected to the color panel display, a problem exists in that color of the color figure on the color panel display cannot be simply changed.